The present article consists of an inquiry into the compatibility of the approach made by John Finnis, in his most famous book Natural Law and Natural Rights, to the problems of Common Good, Justice and Rights - particularly, Human Rights - to the Thomistic doctrine of Natural Law and of Rights. The first section of the article poses a question as to the fidelity of the Thomasian conception of `practical reason' as presented by Finnis, while confronting his translation of Aquinas' excerpt on the subject with the original text, seeking to demonstrate that he departs from the what the author originally meant: that practical and theoretical reason are intertwined. The purpose of the second section is to present a comparison of Finnis's conceptions of Common Good and Justice to the Thomasian view of both subjects, interpreted with regard to Aquinas Metaphysics of Being as intensive act, as well as to his triadic conception of the created Good. After that, in the third part of the article, Finnis's view on Rights and, more specially, of Human Rights, is assessed, having as a paradigm the Thomistic conception of Right (ius), through an interpretation based on his Metaphysics.